Acid halogen electrotinning baths are designed to plate high quality tin deposits on steel sheet or strip at high current densities and high production rates. These baths generally have an anode efficiency of almost 100% and a cathode efficiency of about 85%. The baths are customarily formulated with halogen salts including stannous chloride, sodium chloride, sodium fluoride and sodium bifluoride. High concentrations of chlorides in the bath maintain maximum conductivity. Fluorides are complexing ions which stabilize the bivalent (stannous) tin ion by forming a complex sodium fluostannate II having the general formula Na.sub.2 SnF.sub.6. The bivalent tin ions necessary for plating are supplied by tin anodes augmented by periodic additions of stannous chloride. The acidity of the bath is customarily maintained by periodic additions of hydrochloric acid.
The high speed of the steel sheet or strip in modern high production lines agitates the bath, resulting in air being dissolved or entrapped in the bath. The bivalent tin ion in the bath is oxidized by the oxygen in this air to the quadrivalent (stannic) tin ion in the form of a complex fluoride salt which is only slightly soluble in the bath, and is not suitable for plating. When the amount of salt in the bath exceeds its solubility in the plating bath, it is precipitated in the form of a complex fluoride salt sludge. The major constituent of the sludge is sodium fluostannate IV having the general formula Na.sub.4 SnF.sub.6.
Metallic ions, such as ferrous iron and copper accelerate the oxidation of the bivalent tin ion. It is virtually impossible to prevent the presence of the iron ion in the bath since iron is introduced with the steel sheet or strip both by "drag-in" from the pickling baths and by dissolution from the steel sheet or strip in the acid plating bath. The bivalent (ferrous) iron ion introduced into the bath is also oxidized by the oxygen in the dissolved and entrapped air to the trivalent (ferric) iron ion. The trivalent iron ion is reduced to the ferrous iron ion in the bath. The reaction during the reduction of the trivalent iron ion to the bivalent iron ion accelerates the oxidation of the bivalent tin ion to the quadrivalent tin ion.
The oxidation of the bivalent tin ion reduces the amount of tin available for tinning purposes and significantly reduces the amount of soluble fluoride in the bath. As a consequence, frequent additions of stannous chloride and sodium fluoride and bifluoride must be made to the bath to maintain the concentration of the bath, thereby increasing chemical costs. The accumulation of large quantities of sludge in the tinning tanks makes it necessary to stop the plating line periodically so that the sludge can be removed from the bath. The sludge is removed manually. The termination of the plating operation is necessary for such removal and results in lost production time. Additionally, manual removal of the sludge is costly.
The sludge contains both tin and fluoride, and can be sold for a portion of the original value of the tin. The fluoride is a total loss since generally it is not recovered. The oxidation of the bivalent tin ion to the quadrivalent ion and the resulting formation of sludge is, therefore, costly in terms of lost production time, in loss of chemicals and in decreased tining efficiency.
It is naturally desirable to inhibit the oxidation of the bivalent tin ion to prevent the formation of the complex fluoride salt sludge and the accompanying decrease in efficiency of the plating bath. One prior art method for reducing the formation of sludge is to add sodium ferrocyanide to the bath to combine with the iron ions and form a very slightly soluble sodium ferrous ferrocyanide precipitate. The removal by precipitation of the iron from the bath reduces the amount of fluostannate IV sludge produced in a given time, but does not prevent the oxidation of the bivalent tin ion to the quadrivalent tin ion by oxygen dissolved or entrapped in the bath. Sodium ferrocyanide thus controls sludge formation by reducing the rate of oxidation of the bivalent tin ion but does not prevent the oxidation of the bivalent tin ion.
It is therefore the object of this invention to provide an improved acid halogen electrotinning bath which is less susceptible to oxidation and sludge formation than the baths now in use and in which the problems described above are alleviated.